Neutralizing circuit



June 932:- w. VAN B. ROBERTS ,861,

' NEUTRALI ZING' CIRCUIT Filed Sept. 3; 1927 r--1nr-;1' H E 2L5:

lNVENTOR WALTER VAN B ROBERTS TORNEY Patented June 7, 1932 UNITED STATES PATENT OFFICE WALTER VAN B. ROBERTS, OF PRINCETON, NEW JERSEY, ASSIG-NOR T RADIO CORPORA- TION OF AMERICA, A CORPORATION OF DELAWARE N EUTRALIZING CIRCUIT Application filed September 3, 1927. Serial No. 217,368.

My invention relates to a neutralizing circuit and more particularly it relates to improvements on the neutralizing circuit disclosed by C. 1V. Rice in the United States 5 patent granted to him on March 16, 1920,

By neutralization, it has been attempted in various radio receiving circuits to prevent two effects, first, radiation by an antenna caused by an impulse in an output circuit feeding back energy to an input circuit which in turn is coupled to the antenna, and secondly, to prevent regeneration which causes (sometimes) sustained local generation of oscillations. Of the two effects, only regeneration produces a distorting effect in a receiver.

In bridge neutralizing circuits known at present, blocking action, (that is, perfect prevention of radiation) is maintained where a divided input circuit and a neutralizing condenser is used in connection with a thermionic amplifier. However, for the reason that it is impossible to obtain 100% coupling in the inductances of the divided input circuit,

an absolute zero of regenerative action independent of frequency and tuning has not been obtained.

Consequently, itis an object of this invention to provide a neutralizing circuit in which regenerative distortion is completely absent and in which this adjustment is main tained perfect at the expense of the radiation blocking, in such a manner that, even though radiation blocking is not 100% efficient in my system, radiation will be kept so small that it will not seriously afi'ect nearby receivers. Therefore it can in effect be said 40 that this invention provides a neutralizing system in which regenerative action is completely eliminated and in which radiation blocking is approximated.

Another object of this invention is to provide a neutralizing system in which the neutralizing means are kept constant, that is, to provide a neutralizing system in which regardless of the frequency to which the receiver is tuned, it will not be necessary to vary the value of the neutralizing means in order to maintain complete absence of regenerative effects at the signal frequency.

The manner in which these objects will be carried out will be explained below' with the aid of the accompanying drawing in which Fig. 1 is a wiring diagram of my improved neutralizing circuit; Fig. 2 shoWs Fig. 1 redrawn as a bridge, anc

Fig. 3 shows my improved circuit coupled to a receiving antenna.

Referring to Fig. 1, C L and L indicate a tuned divided input circuit. Voltage variations across part of the tuned circuit indicated by L are applied across the grid and filament of the thermionic tube indicated diagrammatically in Fig. 1, V the filament, grid and plate of which are denoted by the numerals 1, 2 and 3 respectively.

Regeneration in this type of circuit may.

be explained as follows. When an input voltage is applied across the grid 2 and filament 1 of the tube, a plate voltage appears as a voltage drop across the load in the plate circuit. This voltage tends to force 2. capacity current i from plate 3 to grid 2 of the tube. Some of this capacity current 2' will flow through L as indicated, the portion of the current flowing through L being indicated by the numeral 2}. As a result, the current 2', will cause a drop in voltage between the grid 2 and filament 1 of a value equal to 2' Z where Z, represents the react-ance of L thus giving rise to what is known as the undesired regenerative action which causes distorting effects in the receiver telephone.

In order to preventthe regenerative and radiation effects, it has been proposed to form a bridge such as shown in Fig.2 in

which the coupling between the inductance L and the inductance L should be Theoretically, this would accomplish both results. The magnetic fields through L and L1 would Completely cancel, with the result that no difference of potential between grid and filament would be produced by the presence of a plate voltage, and no current would be produced in an antenna coupled to the input circuit. Thus,'regeneration would.

be eliminated and blocking action perfected both at once.

But in practice, it is impossible to get 100% coupling between inductances L and L. In case the bridge is balanced this will be immaterial as far. as radiation blocking is concerned, since as shown in- Fig. 2 if a small current flows in L and L due to the Voltage 6, the voltage drops in L and L will lie-equal, the grid 2 and point A will beat the same potential and therefore there will be no feed-back from the output to its con? jugate input circuit if properly coupled to the two coils.

According to my invention, however, the bridge is deliberatelyunbalanced, thus impairing: the perfection ofblocking action, butsecuring'in its stead complete absence of regenerationv action, and in order to secure this result I make the capacities G and C constant, andI make the coupling between inductances L and L2 zero, by separating them: and by'keeping them at right angles, and I make the ratio of inductance L to inductanceL' equal to K, a constant.

The results of these changesmade accordingxtomyinvention may best -be explained mathematically, particular reference being had to Fig: 1 which indicates the various currents flowing in the various capacities and inductances of the circuit.-

St'a-rting'with the general case where there ismutuaiinductance L between L and L indicating the various impedances-by Z with an: appropriate sub-numeral and applying Kirchofis la=ws to thevariousmeshes we have.

For. zeroregenerati'on, theremust be no potential difference between grid and filament when the plate alterating electro-motive force a is present; That'is, going from point Q to point Y topoint H the alternating electro-motive forces most add up to Zero and therefore for zero regeneration- Undertheseconditions" solving the: six equatrons given above it Wl'll be found that for zeroregeneration value of the neutralizing capacity necessary for zero regeneration when there is mutual inductance between the coils L and L However, remembering that it will be clear that at each frequency to which the input circult is tuned, for zero reand X =21rfL generation, a new valueof neutralizing capacity C, may be required. But if according to my invention the coupling between L and L is made equal to zero. the value of the neutralizing:reactance for zero regeneration becomes which again shows that the value of the neutralizing capacity for zero regeneration when the coupling is zero is again not independent of the frequency.

If however, we consider the frequency to which the circuit is tuned and for which Equation (11) shows that the value of the neutralizin capacity for zero regeneration at the tuned frequency is a constant. provlded that capacities C and C are kept constant,

and provided that the inductance ratio K remains constant. Havlng done this, it can be said that my arrangement of the Ricev circuit with no mutual inductance between the inductances, indicated in. Fig.v 1 by L and L may have zero regeneration at the frequency to which the input circuit is tuned, and theneutralizing capacity which gives this result is the same regardless of the frequency to which the input circuit may be tuned.

Altho by my'system the capacity O must remain constant, the input circuit may be tuned by varying the inductances L and L while maintaining their ratio constant.

The manner in which the zero mutual and the constant ratio of inductance between the inductances L and L is maintained is shown in Fig. 3. Zero coupling is maintained by keeping the coils L and L far apart or shielded or by keeping them at right angles to one another. The inductance ratio is maintained constant by means of a. suitable coupling connector 10 which varies both inductances alike and at the same time varies the natural frequency of the input circuit.

Having thus described my invention what I claim is:

l. A neutralizing bridge circuit for a thermionic tube amplifier comprising a constant capacity in one of the arms of the bridge and between the anode and cathode of the tube, and inductances in other arms of the bridge and in the input circuit of the tube having zero coupling between them whereby regenerative blocking is maintained perfect at all frequencies of oscillations impressed upon the input circuit of the amplifier.

2. A neutralizing bridge circuit comprising conjugate input and output circuits constant capacities and variable inductances in the arms of the bridge, the inductances having zero mutual inductive coupling between them, and a constant capacity in the input conjugate circuit.

3. In a circuit for the amplification of high frequency oscillations a thermionic tube, a tunable divided input circuit comprising a constant capacity and a pair of inductances whose inductance ratio for all tunings of the input circuit is maintained substantially constant and having zero coupling between them, and a substantially constant neutralizing ca pacity connected, between one end of one of the input inductances and the anode of the tube.

4. In a neutralizing bridge circuit utilizing a pair of capacities of constant value in the arms of the bridge, and a divided input circuit, having a capacity of constant value and a pair of inductances having zero mutual inductance, the method of tuning by varying the inductance of the divided input circuit, and simultaneously maintaining the ratio of the inductances in each branch of the divided input circuit constant.

5. A neutralizing bridge circuit for a thermionic tube amplifier comprising a constant capacity in one of the arms of the bridge and between the cathode and anode of the tube, inductances in other arms of the bridge and in the input circuit of said tube having zero coupling between them, and a capacity of constant value in shunt with said inductances.

6. A neutralizing bridge circuit for a thermionic tube amplifier comprising a constant capacity in one of the arms of the bridge and between the anode and cathode of the tube, inductances in other arms of the bridge and in the input circuit of the tube having zero coupling between them, and means for varying said inductances and simultaneously maintaining the ratio between said inductances constant at all frequencies.

7. A combination in a radio receiving system of an electron discharge amplifier having a cathode, anode and grid and having a resonant grid circuit including a pair of inductances shunted by a capacity, a connection between the cathode of said amplifier and a point between said inductances, a connection between the anode of said amplifier and one end of one of said inductances, said inductances having zero coupling between them.

8. The combination in a radio receiving system of an electron discharge amplifier having a cathode, anode and a grid and having a resonant grid circuit comprising a pair of variable inductances shunted by a single constant capacity, a connection between the cathode of said amplifier and a point between said inductances, a connection between the grid of said amplifier and one end of one of said'inductances, and means for varying the tuning of said grid circuit by adjusting said inductances, said inductances having zero mutual inductance.

9. In a neutralizing bridge circuit for a thermionic tube amplifier comprising a pair of capacities of constant value in the arms of the bridge, said amplifier having a divided input circuit comprising a capacitance of constant value and a pair of inductances having zero mutual inductance, and means for tuning the input circuit by varying the inductance of the divided input circuit.

WALTER VAN B. ROBERTS. 

